Hairy-2a promotes floor plate fates at the expense of the notochord, blocking involution in the Xenopus organiser.

SILVIA L. LÓPEZ, MARÍA V. ROSATO SIRI, ALEJANDRA PAGANELLI, PAULA G. FRANCO, PAULA RAIMONDI Y ANDRÉS E. CARRASCO.

San Francisco, USA

Congreso; Society for Developmental Biology, 64th annual meeting; 2005

SDB

Hairy-2a Promotes Floor Plate Fates at the Expense of the Notochord, Blocking Involution in the Xenopus Organiser.   Silvia L. Lo´pez, Marý´a V. Rosato-Siri, Paula G. Franco, Alejandra R. Paganelli, Paula Raimondi, and Andre´s E. Carrasco. LEM-IBC.   School of Medicine, UBA-CONICET, Buenos Aires, Argentina.   In Xenopus embryos, the midline neural plate cells that later become the floor plate (FP) conform the notoplate, which arises from the dorsal non-involuting marginal zone (DNIMZ), a region of ectoderm just above the dorsal involuting marginal zone (DIMZ). The latter contains mesodermal cells that enter through the blastopore during gastrulation, giving rise to the prechordal plate and notochord. The notoplate precursors remain in the ectodermal layer and undergo convergent-extension movements, laying the future FP along the A–P axis. The Xenopus organiser contains cells equally potent to develop as notochord or FP. Before midgastrula, Notch executes a binary cell-fate switch that favours FP development at the expense of the notochord (Lo´pez et al., Development 130, 2225– 2238, 2003). Now, we found that the Notch-target gene hairy- 2a is expressed in an arc within the DNIMZ and in scattered cells in the DIMZ, where delta-1 transcripts are also scattered. Hairy-2a+ cells gradually converge and extend along the A–P axis, forming the notoplate. We propose that Delta-1 is the ligand that triggers the binary cell-fate switch by activating Notch in the surrounding cells within the DIMZ. Notch, in turn, activates hairy-2a, which impedes the involution of cells through the blastopore, promoting their incorporation into the population of notoplate precursors in the DNIMZ.